1. Field of the Invention
The present invention relates to a wavelength conversion apparatus which uses a wavelength conversion element and a semiconductor laser device as an excitation light source.
2. Description of the Related Art
Conventional wavelength conversion apparatuses use laser optical crystals such as Nd-doped YAG, Nd-doped YVO4, Nd-doped YLF, or the like. Each of the laser optical crystals is excited by a semiconductor laser device which emits laser light having a wavelength band broader than an absorption band of the laser optical crystal, and a wavelength of the laser light emitted by the laser optical crystal is converted by a nonlinear crystal or a wavelength conversion element such as domain-inverted LiNbO3. As a result of the conversion, for example, the wavelength of the laser light emitted by the laser optical crystal is reduced to half.
Typically, a laser optical crystal of YAG is excited by excitation laser light having a wavelength of 809 nm, which is emitted by a wide-beam semiconductor laser device, and thus laser light having a wavelength of 946 nm is emitted. Then, blue light having a wavelength of 473 nm is obtained as a second harmonic wave by a wavelength conversion element made of, for example, domain-inverted LiNbO3. However, since mode control is insufficient in the wide-beam semiconductor laser device, mode hopping occurs. Therefore, light output power fluctuates even under a constant driving current, and an unignorable amount of noise (represented by a ratio of RF noise intensity and DC noise intensity of the optical output) is produced. In addition, even in the longitudinal mode, the full width at half maximum of the oscillation wavelength of the wide-beam semiconductor laser device increases with increase in the light output power. For example, the full width at half maximum exceeds 2 nm even if the light output power is in the range of 200 to 500 mW.
Nevertheless, the absorption coefficient of the YAG crystal for the light of 809 nm is small, and the absorption band including the wavelength of 809 nm is narrow (2 nm). In addition, due to the noise of the semiconductor laser device, the output laser light having the wavelength of 946 nm also includes noise. Further, due to the mode hopping in the wide-beam semiconductor laser device, wavelengths of a portion of the output laser light of the wide-beam semiconductor laser device are not within the absorption band of the YAG crystal, and such a portion of the laser light also increases noise. Furthermore, the noise in the laser light of 946 nm causes noise in the blue light output from the wavelength conversion element.
In addition, the above narrow absorption band of the laser optical crystal must be adjusted to the wavelength band of the output of the semiconductor laser device in the conventional wavelength conversion apparatus. Therefore, precise temperature control is necessary in order to achieve the adjustment.
Further, since the laser optical crystals utilize transitions in doped rare earth elements, only limited wavelengths can be obtained from the laser optical crystals. Therefore, the second harmonic waves obtained by converting the wavelengths of the laser light emitted by the laser optical crystals also have only limited wavelengths.
Furthermore, when modulation of the second harmonic wave is required, the modulation may be realized by directly modulating the semiconductor laser device as the excitation light source, instead of modulation of the modulation of the second harmonic wave per se by using an acoustooptic (AO) modulator or the like. In the case of the direct modulation of the semiconductor laser device, the width of oscillated wavelength of the semiconductor laser device is readily broadened to be more than 2 nm, i.e., greater than the absorption band of the YAG crystal as the laser optical crystal. Therefore, the efficiency is lowered. In addition, since the optical output of the semiconductor laser device becomes a broadened longitudinal mode, the output optical power varies due to mode competition. Thus, according to the conventional technique, the modulated, wavelength-converted light cannot be made stable and the conventional wavelength conversion apparatuses are not efficient.
As described above, since the conventional wavelength conversion apparatuses use laser optical crystals having narrow absorption bands, output light of excitation light sources cannot be utilized efficiently, and noise is produced. Therefore, the conventional wavelength conversion apparatuses are not suitable for obtaining stable optical output on a long-term basis. In addition, according to the conventional wavelength conversion apparatuses, obtainable wavelengths are limited by the laser optical crystals used in the wavelength conversion apparatuses, and therefore it is impossible to obtain an arbitrary wavelength in the visible and ultraviolet ranges.
The object of the present invention is to provide a wavelength conversion apparatus which can efficiently output stable laser light having an arbitrary wavelength on a long-term basis, without the need for precise temperature control.
According to the present invention, there is provided a wavelength conversion apparatus containing an excitation light source which includes a semiconductor laser device having an active layer, and emits first laser light; an optical amplifying element which is excited by the first laser light to emit second laser light; a resonator formed by the optical amplifying element and a mirror arranged outside the optical amplifying element; and a wavelength conversion element arranged in the resonator. In the wavelength conversion apparatus, the optical amplifying element is a semiconductor optical amplifying element having an active layer on a substrate thereof.
The wavelength conversion apparatus according to the present invention uses the semiconductor optical amplifying element having an active layer on a substrate thereof, instead of the laser optical crystal. Since, generally, absorption bands of semiconductor optical amplifying elements are broader than the wavelength band of the semiconductor laser device as the excitation light source, and absorption coefficients of the semiconductor optical amplifying elements are greater than those of the laser optical crystals, the semiconductor optical amplifying element in the wavelength conversion apparatus according to the present invention can efficiently absorb the excitation light (first laser light) which is output from the semiconductor laser device. Therefore, a stable optical output can be obtained from the wavelength conversion apparatus according to the present invention, and noise in the optical output is reduced.
In addition, in the wavelength conversion apparatus according to the present invention, precise temperature control is unnecessary, while such precise temperature control is necessary in the conventional wavelength conversion apparatus for adjusting the absorption band of the laser optical crystal to the oscillation wavelength band of the semiconductor laser device.
When semiconductor material forming the active layer of the semiconductor optical amplifying element is appropriately chosen, and the composition of the semiconductor material is appropriately controlled, for example, as described later in paragraph (5) of the additional features, the semiconductor optical amplifying element can emit laser light having an arbitrary wavelength in the range from 750 nm to 1,400 nm. Therefore, an optical output having an arbitrary wavelength in the range from 375 nm to 700 nm can be obtained by the wavelength conversion element as a second harmonic wave.
Further, since, in the wavelength conversion apparatus according to the present invention, the semiconductor optical amplifying element is used instead of the laser optical crystal, stable modulated laser light can also be obtained as the second harmonic wave even when the modulation is realized by directly modulating the semiconductor laser device as the excitation light source.
The wavelength conversion apparatus according to the present invention may have the following additional features:
(1) In the above wavelength conversion apparatus according to present invention, the wavelength of the first laser light is preferably shorter than the wavelength of the second laser light.
(2) In the above wavelength conversion apparatus according to present invention, the mirror may be a concave mirror, and is preferably arranged so that the focal plane of light reflected by the mirror is located inside the semiconductor optical amplifying element.
(3) In the above wavelength conversion apparatus according to present invention, the active layer of the semiconductor laser device is preferably made of at least one of AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa60.4), GaAsxP1xe2x88x92x (0xe2x89xa6xxe2x89xa60.4), InxGa1xe2x88x92xAsyP1xe2x88x92y (0xe2x89xa6xxe2x89xa60.4, 0.7xe2x89xa6yxe2x89xa61), InxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa60.5), and InxAlyGa1xe2x88x92xxe2x88x92yAs (0xe2x89xa6xxe2x89xa60.5, 0xe2x89xa6yxe2x89xa60.4).
(4) In the above wavelength conversion apparatus according to present invention, the stripe width of the active layer of the semiconductor laser device is preferably at least 3 xcexcm.
(5) In the above wavelength conversion apparatus according to present invention, the active layer of the semiconductor optical amplifying element is preferably made of at least one of direct gap semiconductor crystals of AlxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa60.4), GaAsxP1xe2x88x92x (0xe2x89xa6xxe2x89xa60.4), InxGa1xe2x88x92xAsyP1xe2x88x92y (0xe2x89xa6xxe2x89xa60.4, 0.7xe2x89xa6yxe2x89xa61), InxGa1xe2x88x92xAs (0xe2x89xa6xxe2x89xa60.5), and InxAlyGa1xe2x88x92xxe2x88x92yAs (0xe2x89xa6x xe2x89xa60.5, 0xe2x89xa6y0.4), InP, and InAlGaAsP.
(6) In the above wavelength conversion apparatus according to present invention, at least one active layer, including the above-mentioned active layer of the semiconductor optical amplifying element, is preferably arranged at a location in which electric field strength of a standing wave generated in the semiconductor optical amplifying element is great.
(7) In the above wavelength conversion apparatus according to the present invention, a dielectric film may be provided on a light-exit end surface of the semiconductor optical amplifying element.
(8) In the wavelength conversion apparatus described in the above paragraph (7), the dielectric film is preferably made of at least one of SiO2, Al2O3, TiO2, SiN4, and Si.
(9) In the wavelength conversion apparatus described in the above paragraph (7), the dielectric film may be formed by ECR (electron cyclotron resonance) sputtering.
(10) In the above wavelength conversion apparatus according to present invention, the active layer of the semiconductor optical amplifying element is preferably located only in a portion which the first laser light enters, and has a structure for blocking the first laser light from portions other than the above portion which the first laser light enters.
(11) In the above wavelength conversion apparatus according to present invention, the active layer of the semiconductor optical amplifying element has at least one quantum well structure.
(12) In the above wavelength conversion apparatus according to present invention, a portion of the semiconductor optical amplifying element other than the active layer thereof is preferably doped with impurity of at least 5xc3x971017 cmxe2x88x923 to make the portion a p type or an n type.
(13) In the above wavelength conversion apparatus according to present invention, the semiconductor optical amplifying element may have a periodic structure on a side, opposite to a light-exit end surface, of the active layer of the semiconductor optical amplifying element, where the periodic structure reflects light generated by the semiconductor optical amplifying element with high reflectance.
(14) In the wavelength conversion apparatus described in the above paragraph (13), the periodic-structure is preferably made of at least two semiconductor materials having different refraction indexes, and optical absorption coefficients of the at least two semiconductor materials are at most 5xc3x97102 for a wavelength of light generated in the semiconductor optical amplifying element.
(15) In the above wavelength conversion apparatus according to present invention, the semiconductor optical amplifying element may be supplied with the first laser light from the substrate side of the semiconductor optical amplifying element, and emits the second laser light from the side opposite to the substrate in a direction perpendicular to the substrate.
(16) In the wavelength conversion apparatus described in the above paragraph (15), the semiconductor optical amplifying element may have a dielectric film on a light-entrance side of the semiconductor optical amplifying element.
(17) In the wavelength conversion apparatus described in the above paragraph (15), a reflectance of a light-entrance end surface of the semiconductor optical amplifying element is preferably at most 30% at a wavelength of the first laser light, and at least 50% at a wavelength of the second laser light.
(18) In the wavelength conversion apparatus described in the above paragraph (15), a reflectance of a light-exit end surface of the semiconductor optical amplifying element is preferably at most 30% at a wavelength of the second laser light.
(19) In the above wavelength conversion apparatus according to present invention, the semiconductor optical amplifying element may emit the second laser light from a surface of the semiconductor optical amplifying element in a direction perpendicular to a substrate, and the first laser light may enter the semiconductor optical amplifying element from the same surface of the semiconductor optical amplifying element.
(20) In the wavelength conversion apparatus described in the above paragraph (19), a reflectance of the above surface of the semiconductor optical amplifying element is preferably at most 30% at a wavelength of the first laser light, and at most 30% at a wavelength of the second laser light.
(21) In the wavelength conversion apparatus described in the above paragraph (19), the first laser light may enter the semiconductor optical amplifying element in a slanting direction with respect to the substrate of the semiconductor optical amplifying element.
(22) The wavelength conversion apparatus described in the above paragraph (19) may further contain a beam splitter on an optical path of the second laser light, where the beam splitter may supply the first laser light to the semiconductor optical amplifying element in a direction perpendicular to the substrate of the semiconductor optical amplifying element.
(23) In the wavelength conversion apparatus described in the above paragraph (21), the first laser light is preferably incident on a light-entrance end surface of the semiconductor optical amplifying element as a substantially p-polarized light.
(24) In the wavelength conversion apparatus described in the above paragraph (21), an incident angle of the first laser light on a light-entrance end surface of the semiconductor optical amplifying element is preferably within xc2x110 degrees of the Brewster""s angle.
(25) The above wavelength conversion apparatus according to present invention may further contain a filter in a common optical path of the second laser light and light which is preferably reflected by the mirror and converted by the wavelength conversion element.
(26) In the wavelength conversion apparatus described in the above paragraph (25), the filter is preferably one of a birefringent filter, an etalon, and a dielectric filter.
(27) In the above wavelength conversion apparatus according to present invention, a reflectance of a light-entrance end surface of the wavelength conversion element is preferably at most 30% at a wavelength of the first laser light.
(28) In the above wavelength conversion apparatus according to present invention, a reflectance of a light-exit end surface of the wavelength conversion element is preferably at most 30% at a wavelength of wavelength-converted light which is generated by conversion by the wavelength conversion element.
(29) In the above wavelength conversion apparatus according to present invention, the semiconductor laser device may be driven by applying a direct current and a high-frequency current thereto.
(30) In the above wavelength conversion apparatus according to present invention, the semiconductor laser device may be driven by applying a pulsed current thereto.
(31) In the above wavelength conversion apparatus according to present invention, the semiconductor laser device may be driven by applying a direct current and a pulsed current thereto.
(32) In the above wavelength conversion apparatus according to present invention, the semiconductor optical amplifying element may be caused to emit the second laser light by applying a current thereto from outside, in addition to supply of the first laser light thereto.